Electricity Generation

Methane Digesters (Small)

Agricultural, industrial, and human digestion processes create an ongoing (and growing) stream of organic refuse. Without thoughtful management, organic wastes can emit fugitive methane gases as they decompose. Methane creates a warming effect 34 times stronger than carbon dioxide over one hundred years.

One option is to control decomposition of organic waste in sealed tanks called anaerobic digesters. They harness the power of microbes to transform scraps and sludge and produce two main products: biogas, an energy source, and solids called digestate, a nutrient-rich fertilizer. The digestion process unfolds continuously, so long as feedstock supplies are sustained and the microorganisms remain happy.

Anaerobic digestion is used in backyards and farmyards around the world, and that use is on the rise. Small-scale digesters dominate in Asia. More than 100 million people in rural China have access to digester gas, which is used for cooking, lighting, and heating. In fact, during his years in ancient China, Marco Polo encountered covered sewage tanks that produced cooking fuel.

Biogas can reduce demand for wood, charcoal, and dung as fuel sources and therefore their noxious fumes, which impact both planetary and human health. Digestate enriches home gardens and small agricultural plots.

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Rank and Results by 2050

1.9 gigatonsreduced CO2

$15.5 Billionnet implementation cost

$13.9 Billionnet operational savings

Impact: We project that by 2050, small digesters can replace 57.5 million inefficient cookstoves in low-income economies, resulting in 1.9 gigatons of carbon dioxide emissions avoided at a cost of $15.5 billion. If you couple this impact with that of large methane digesters, the cumulative result is 10.3 gigatons of carbon dioxide emissions avoided.

Anaerobic digestion is a biological process that produces a gas (biogas) principally composed of methane and carbon dioxide. Methane digesters come in a variety of different tank designs to capture methane and combust it to create heat. This reduces greenhouse gas emissions from the use of fossil fuels or solid biomass fuel, while also mitigating fugitive methane and nitrous oxide emissions from the anaerobic breakdown of manure. Through global adoption of methane digesters (small) from 2020-2050, around 57.5 million inefficient biomass/charcoal/fossil fuel-based cookstoves can be replaced, avoiding 2.10 gigatons of carbon dioxide-equivalent emissions.

In the Drawdown model, the manure fed into small methane digesters is from the cattle owned by the household so there is no fuel cost, but some maintenance is required.

Methodology

To derive mitigation impact results and financial considerations for methane digesters (small), several steps were taken. First, a forecast was calculated for the total use of traditional cookstoves from 2014-2050, measured in terawatt-hour therms. Then, current adoption [1] of small methane digesters was determined, future adoption scenarios were forecast for that period, an emissions mitigation value was derived in terawatt-hour therms per digester per year, and the emissions mitigation and costs were calculated compared to a Reference Scenario that keeps methane digester adoption at its current percentage of global solid fuel cookstove adoption.

Global data for the total addressable market for methane digesters (small) was obtained from two International Energy Agency sources (IEA, 2006 and 2013), and constructed from others by means of calculated conversions. Assumptions were derived from the literature review for the following variables: population growth; average population per household; average household energy use for cooking per capita per year; a weighted average energy efficiency factor for stove and fuel type mix; and the percentage of the population using solid fuels. These assumptions were then used to calculate market values for Asia (sans Japan), Latin America, and the Middle East and Africa, and were aggregated for a global set of values from 2014-2060. These values, along with IEA interpolated values and extrapolations, were assessed, and best-fit trend lines and values were selected for a composite total addressable market.

Current adoption and future adoption scenarios for methane digesters (small) were created by: taking the population of cattle, buffalo, and pigs for each country in Asia (sans Japan), Africa, the Middle East, and Latin America from the Food and Agriculture Organization (FAO)’s 2014 database; estimating a certain manure production per day (minus a feasibility factor of what is collectable); then factoring in an average daily manure requirement for a functioning digester. The result was a total potential capacity for small methane digesters per each region. Current data for 2014 on the number of installed digesters was used to derive current adoption.

Impacts of increased adoption of methane digesters (small) from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.

Plausible Scenario: For this scenario, it is assumed that each region in question achieves the current adoption in the region Asia (sans Japan): 36.5% of total possible capacity. This results in 158 terawatt-hour therms of adoption by 2050, or 61,481,609 small methane digesters.

Drawdown Scenario: This scenario assumes that each region achieves the current adoption in China—52.64% of the total possible capacity—which results in 173 terawatt-hour therms of adoption by 2050 (67,379,863 small digesters).

By avoiding methane and nitrous oxide emissions through the use of small methane digesters and the replacement of inefficient fuelwood cookstoves, each digester reduces around 1.25-2.95 tons of carbon dioxide-equivalent emissions per year (Zhang et al., 2013; Izumi et al., 2016). Because of leakages and biogas burning, emissions from small methane digesters amount to around 0.02 kilograms of carbon dioxide per mega joule of cooking energy produced (Zhang et al., 2013). No substantial double-counting or integration effects were factors in generating the results for this solution.

Financial Model

The cost of small methane digesters depends on the country of installation, the size, the design, and the material used, and ranges from US$300 in Costa Rica to US$1200 in Kenya. [4] The operational cost of small methane digesters is very low as the manure is almost free of cost, but there is a need for regular maintenance. Maintenance costs ranging from US$0.01-0.08 per kilowatt-hour (Ghimire, 2005 and Shailendra et al., 2015). High initial capital costs, improper construction leading to high maintenance or repair costs, and inadequate operator training are the main barriers to large-scale adoption of small methane digesters (GACC, 2016).

Most of the cost of traditional stoves has been obtained from sources such as the World Bank and the US Environmental Protection Agency (EPA). Weighting based on cooking fuel mix was applied to all costs.

Results

The total carbon dioxide-equivalent reductions that can be achieved through the use of small methane digesters from 2020-2050 in the Plausible Scenario are about 1.9 gigatons, with a cumulative first cost of US$28.8 billion to implement 61.5 million digesters by the year 2050. The Drawdown Scenario shows a mitigation of 2.6 gigatons from 2020-2050, and the Optimum Scenario shows a mitigation of 9.8 gigatons at a cost of US$55.1 billion.

Discussion

Small methane digesters have the potential to provide reliable, carbon emissions-mitigating household heating across the world for cooking and other uses. The major risk associated with adoption is disrepair and fugitive emissions. Creative feedstock switching and community-scale aggregating are in development to increase the overall technical feasibility, which is currently the limiting factor for the technology.

[1] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.

[2] For more about the Total Addressable Market for the Energy Sector, click the Sector Summary: Energy link below.